Stream flow velocity alters submerged macrophyte morphology and cascading interactions among associated invertebrate and periphyton assemblages

• Morphological responses of Myriophyllum spicatum to stream flows are described.
• Growth of periphyton on macrophyte at different water velocities studied.
• Macroinvertebrate assemblages on macrophyte beds were investigated.

Submerged macrophytes play a key role in the functioning of stream ecosystems since they strongly affect the biological and physical environment of the habitat. On the other hand, flow velocity may affect growth and establishment of submerged macrophytes in streams and rivers. However, little attention has been paid to the morphological responses of submerged macrophytes to different stream flows and in the present study we investigate the intraspecific difference in flow adaptation of a common submerged macrophyte, Myriophyllum spicatum L. We found no difference in length of main shoot or total length of lateral shoots of M. spicatum plants grown at high and low stream flow. However, shoot and root dry weight biomass, number of lateral shoots, degree of branching and stem diameter of the main shoot increased significantly with increasing water velocity. In contrast, the opposite trend was observed for leaf whorl area and distances between the internodes of the main shoot. The amount of periphytic algae also decreased with increased water velocity, whereas the macroinvertebrate abundances were nine fold higher at high than at low stream flow, suggesting that grazing may, besides higher stream flow, have been a process behind the lower periphyton growth at high flow. Hence, stream flow not only acts as a stress factor leading to morphological changes in submerged macrophytes, but also induces cascading trophic interactions among periphytic algae and invertebrate assemblages, thereby being a major force in shaping the organism communities of streams and rivers.